A thin film transistor (TFT) having a semiconductor layer (a channel layer) made of an oxide semiconductor such as indium gallium zinc oxide or tin oxide is characteristic in that the operating speed is fast compared to that of a TFT that takes a non-crystalline silicon layer as a semiconductor layer, and in that a crystallization step is not required in contrast to a TFT that takes a polycrystalline silicon layer as a semiconductor layer. Accordingly, in recent years, development of TFTs having a semiconductor layer made of an oxide semiconductor (an oxide semiconductor layer) is being actively pursued.
Like the TFT that takes a non-crystalline silicon layer as a semiconductor layer, the TFT that takes an oxide semiconductor layer as a semiconductor layer is manufactured by various steps of photolithography. Steps of photolithography includes cleaning of a substrate, deposition, resist coating, prebaking, exposure, development, post-baking, etching, resist removal, and the like.
When TFTs that take an oxide semiconductor layer as a semiconductor layer are manufactured by photolithography, characteristics of the TFTs may be varied. Accordingly, Japanese Laid-Open Patent Publication No. 2009-099944 discloses an invention for reducing the variance in the characteristics of the TFTs caused by non-uniform damages incurred to the semiconductor layers at the time of formation of protection layers. According to the invention disclosed in Japanese Laid-Open Patent Publication No. 2009-099944, a semiconductor layer includes a first layer which functions as a channel layer, and a second layer having higher resistance than the first layer (the second layer is provided on the protection layer side of the semiconductor layer). Because the second layer having high resistance is provided at an upper layer of the first layer functioning as a channel layer, the influence exerted on electrical conduction characteristics of a channel region of the TFT by the step of forming a protection layer is made small, and occurrence of variance in the characteristics of the TFTs may be suppressed.